Method of growing crystals by shifting the equilibrium of chemical complexes

ABSTRACT

A METHOD FOR EFFECTING THE SOLUTION GROWTH OF SINGLE CRYSTALS FROM INSOLUBLE CRYSTAL FORMING MATERIALS. THE METHOD INCLUDES FORMING A SOLUTION OF THE CRYSTAL GROWING MATERIAL BY FIRST COMPLEXING THE MATERIAL TO RENDER IT SOLUBLE FOLLOWED BY THE DECOMPOSITION OR DEACTIVATION OF THE COMPLEXED MATERIAL. THIS ALLOWS THE DESIRED CRYSTAL GROWING MATERIAL TO PRECIPITATE OUT OF SOLUTION IN SINGLE CRYSTAL FORM. DECOMPOSITION IS ACCOMPLISHED BY HEATING THE COMPLEXED SOLUTION AT A TEMPERATURE AND FOR A PERIOD OF TIME SUFFICIENT TO SHIFT THE EQUILBRIUM OF THE CHEMICALLY COMPLEXED MATERIAL AND RENDER IT INSOLUBLE.

United States Patent 3,671,200 METHOD OF GROWING CRYSTALS BY SHIFTING THE EQUILIBRIUM OF CHEMICAL COMPLEXES Alton F. Armington, Lexington, and John J. OConnor,

Arlington, Mass, assignors to the United States of lAmerica as represented by the Secretary of the Air orce No Drawing. Filed May 6, 1969, Ser. No. 822,322 Int. Cl. B01j 17/04; C01g 13/00 US. Cl. 23300 4 Claims ABSTRACT OF THE DISCLOSURE A method for effecting the solution growth of single crystals from insoluble crystal forming materials. The method includes forming a solution of the crystal growing material by first complexing the material to render it soluble followed by the decomposition or deactivation of the complexed material. This allows the desired crystal growing material to precipitate out of solution in single crystal form. Decomposition is accomplished by heating the complexed solution at a temperature and for a period of time suflicient to shift the equilibrium of the chemically complexed material and render it insoluble.

BACKGROUND OF THE INVENTION The present invention relates to a method for growing crystals. More particularly, this invention concerns itself with a method for forming crystals by shifting the equilibrium of chemical complexes.

The formation of crystalline materials can be accomplished by resorting to a variety of methods well known in the crystal growing art. One such method involves recrystallization or growth from a solution. Normally, solution growth is performed in one of two ways. In the first method, a nearly saturated solution is slowly evaporated until it becomes saturated and finally supersaturated causing precipitation to occur in crystalline form on a seed. An alternate method involves the slow cooling of a near saturated solution, again resulting in supersaturation and crystal growth. Since both of these methods depends on saturation, a moderately soluble material must be used if crystals of a large size are to be obtained. This constitutes a serious disadvantage with these methods since many materials with important commercial applications are insoluble in conventional solvents.

For example, many materials which are of particular value for use in optical and electronic device (such as the II-VI compounds) are insoluble in normal solvents. However, although they are usually insoluble, they can be dissolved in conventional solvents if they are first chemically complexed, using chelating or other complexing agents. Unfortunately these complexing agents are often strongly bonded to the solute to be recrystallized and the usual methods of recrystallization involving saturated solutions are not efiective for producing large single crystals.

With the present invention, however, a technique has been developed to overcome the problems prevalent with prior art solution growing methods. This technique is accomplished by slowly decomposing or deactivating the complexing agent thus causing a shift in the chemical equilibrium between the complex and the uncomplexed solute. The shift in equilibrium breaks down the complex and results in the slow precipitation of the desired material in single crystal form. As a consequence, crystal growth from solution is accomplished for materials that are usually considered insoluble.

Patented June 20, 1972 ice In accordance with the present invention, it has been found that the solution growth of crystals of relatively insoluble materials can be accomplished by a method which involves complexing the insoluble crystal growing materials and then causing a shift in the chemical equilibrium between the complex and the uncomplexed solute by decomposing the complexed material. Decomposition returns the material to its insoluble state with the result that the crystal growing material slowly pre cipitates out of solution into the desired single crystal form.

Accordingly, the primary object of this invention is to provide a method for growing single crystal structures using a solution technique.

Another object of this invention is to provide a method for growing single crystals and heterotaxial deposits of highly insoluble materials.

Still another object of this invention is to provide a method for growing crystals by shifting the equilibrium of chemically complexed materials.

The above and still further objects and advantages of this invention will become apparent upon consideration of the following detailed description thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Pursuant to the objects of this invention, the present method of growing a crystalline structure involves the steps of chemically complexing a relatively insoluble crystal growing material to form a complexed material, decomposing the complex thereby allowing the crystal growing material to precipitate out of solution in the form of a single crystal. The decomposition of the complex causes a shift in the equilibrium between the complex and the uncomplexed solute, and it is this shift that permits the solution growth of single crystals from relatively insoluble materials.

In general, any relatively insoluble crystal forming material can be used with the method of this invention provided it can be chemically complexed to render it soluble in conventional solvents such as water. In some instances, such as in the case of growing cinnabar crystals, the pH of the solution should be basic to prevent the formation of undesirable ions. This is accomplished by adding a strong base, such as sodium hydroxide, to the solution. Decomposition of the complex is then carried out by heating, oxidation or any other suitable method for breaking down the complex to effect precipitation of the crystalline material.

Among some of the specific crystal materials found to be useful with the present invention are mercury sulfide (cinnabar), tin sulfide, antimony sulfide, mercury selenide, arsenic trisulfide and arsenic pentasulfide. These materials form complexes with sodium sulfide as the complexing agent. Ammonium sulfide also can be utilized as a complexing agent except with the mercury salts.

The growth of cinnebar crystals typifies the problems inherent in previous attempts to growcrystals from relatively insoluble materials. Cinnebar, or mercury sulfide, when grown at high temperatures, forms metacinnabar which slowly converts to cinnebar on cooling. This phase transition (around 344 C.) results in the formation of imperfect crystals. However, by using the solution growth technique of this invention, cinnebar crystals can be produced without going through the phase transition.

In the method of this invention, mercury sulfide is formed and complexed by an excess of sulfide ion in a solution 0.3 molar in mercuric chloride, 1.0 molar in sodium sulfide and 1.0 molar in sodium hydroxide. The sodium hydroxide is added to prevent the formation of the hydrosulfide ion (HS-) which, unlike the sulfide ion (8:), will not hold the mercury sulfide in solution. After filtering, the solution is placed in an open crystallizer dish at room temperature. A cadmium sulfide seed is suspended in the dish. Cadmium sulfide is used since it is insoluble in the complexing media and has the same hexagonal crystalline structure as cinnebar.

A possible explanation of the chemical reaction involved with this method is shown by the fact that the sulfide ion, in contact with air, undergoes oxidation according to the reaction:

with the result that the sulfide ion is slowly removed from the system. In time the excess sulfide is used up and the mercury sulfide precipitates out of the solution. The decreasing sulfide concentration causes the complex to decompose since the sulfide in the complex must remain in equilibrium with the sulfide ion in the solution.

In the system of this invention, the precipitate forms in two areas. First, approximately twenty percent deposits heterotaxially on the seed directly as cinnebar. The remainder of the precipitate falls to the bottom of the crystallizer in the black form. This usually reverts to the red form in about a day, but the resulting crystals are rarely more than a half mm. in size. n the seed, x 3 x 3 mm. crystals are grown out of the heterotaxy within a week. X-ray examination of the resulting crystals confirmed the cinnebar structure. Cinnebar seeds can be used in place of the cadmium sulfide but the time of addition must be carefully controlled to prevent dissolution of the seed in excess sulfide ion. It should be noted, however, that crystals will form without the assistance of a seed crystal, although the use of a seed is preferable.

The temperature of the reaction can be varied from about twenty degrees centigrade to about seventy degrees centigrade. The reaction is slow at room temperature but the deposit is more uniform. Room temperature growth is the more useful if a heterotaxial junction between the cinnabar and cadimum sulfide is desired. The reaction is satisfactory at temperatures above 55 C. but the best operating temperature was found to be in the 30-50 C. range. It should be emphasized also that care must be taken to prevent the solution from drying up, but that the amount of water present is unimportant since the method is not based on solubility changes.

Finally, while the discussion of this invention has been limited to II-VI compounds, it should be obvious that a wide variety of crystals can be grown by using other complexes. However, there are three necessary requirements which must be observed. First, a complexing agent must be found that will dissolve at least a moderate amount of the normally insoluble crystal growing material. Secondly, a seed material, if used, must approximate the structure of the resultant single crystal. This is particularly necessary where there is the possibility of a phase change near room temperature. Finally, a method must be found to slowly decompose or deactivate the complexing agent, thus allowing recrystallization to take place.

With the foregoing general discussion in mind, there is presented detailed examples which will illustrate to those skilled in the art the manner in which this invention is carried out. However, the examples are not to be construed as limiting the scope of the invention in any way.

Example 1 A solution consisting of 1500 cc. water, 60 grams sodium hydroxide (1.0 molar), 172 grams mercury chloride (0.4 molar) and 540 grams of hydrate of sodium sulfide (1.5 molar) was placed into a beaker and stirred to insure that all ingredients were in solution. The solution was then heated to about 45 C. and maintained at that temperature for a period of about one month. Raising the reaction temperature to about 60 C. would have shortened the reaction time, but the preferred operating temperature is in the range of from about 45 to 50 C.

The time of heating is dependent also on the molar proportions of the solution ingredients. Increasing the concentration of the mercury chloride to its saturation point decreases the overall reaction time to about three weeks. The amounts of sodium hydroxide and sodium sulfide also may be varied provided enough sodium hydroxide is used to keep the solution basic and enough sodium sulfide is used to provide an excess of sulfide ion at the beginning of the reaction. At the conclusion of the reaction, single crystals of cinnabar, 2 mm. thick, had precipitated out of solution. X-ray analysis confirmed the presence of the cinnabar structure.

Example 2 A mixture of 157 grams of sodium sulfide and 16 grams of sodium hydroxide was placed in a beaker with 400 cc. of water and stirred to form a solution. Mercury chloride was then added to the solution in increments of about 20 grams until a total of 82 grams had been added. The solution was covered and stirred for 72 hours. The mercury sulfide which was not dissolved formed a sludge in the bottom of the beaker. The saturated mercury sulfide solution was decanted off and placed into another beaker. A cadmium sulfide seed crystal was added to the saturated solution which, in turn, was heated to about 45 C. After three hours, the cadmium sulfide seed had a deposit of mercury sulfide on it. The cadmium seed was then removed and replaced with a mercury sulfide (cinnabar) seed. The reaction was allowed to continue for three weeks, resulting in the growth of a inch cube-shaped single crystal of cinnabar. The use of a cinnabar seed produces optimum results although growth would have continued on the cadmium sulfide crystal since it possesses the same crystal lattice matching.

From a consideration of the foregoing, it can be seen that the present invention provides an improved method for effecting the solution growth of crystals from materials which are relatively insoluble. The growth of single crystals and heterotaxial deposits of highly insoluble materials, such as the sulfides, selenides and tellurides of mercury, tin, antimony and arsenic is accomplished by shifting the equilibrium of a chemically complexed material. In addition to the I-VI compounds discussed above, a wide variety of other crystals can be grown using other complexes provided three requirements are observed. First, a complexing agent must be used that will dissolve at least a moderate amount of the material desired. Secondly, a seed material, if used, must approximate the structure of the resultant single crystal material. Finally, a technique for slowly decomposing or deactivating the complexing agent must be utilized in order for recrystallization to take place.

We claim:

1. A method for the solution growth of single crystals from insoluble materials comprising the steps of (1) forming a solution of a normally insoluble crystal growing material composed of a compound having one moiety selected from the group consisting of mercury, tin, antimony and arsenic and the other moiety selected from the group consisting of sulfur, selenium and tellurium by chemically complexing the said insoluble material with an ionic complexing agent having excess ions which are compatible with said insoluble material and capable of forming a chemical complex therewith to render said material soluble and (2) heating the complexed material to a temperature and for a period of time sufficient to decompose the complexed material and effect a shift of the chemical equilibrium between the complex material and the uncomplexed solute thereby allowing the said crystal growing material to crystallize out of solution in a single crystal form.

2. A method for the solution growth of single crystals from insoluble materials comprising the steps of forming an aqueous solution composed of (1) a compound having one moiety selected from the group consisting of mercury,

tin, antimony and arsenic and one moiety selected from the group consisting of sulfur, selenium and tellurium, (2) a complexing agent having excess ions which are compatible with said compound and capable of forming a chemical complex therewith, and (3) a strong base; heating said aqueous solution to a temperature of from about 20 C. to 70 C.; maintaining the temperature of said solution for a period of time suflicient to decompose the complexed compound, thereby allowing the said compound to crystallize out of solution in a single crystal form.

3. A method in accordance with claim 2 wherein said solution comprises mercury chloride, sodium sulfide and sodium hydroxide and said solution is heated to a temperature of about 45 C. for a period of about one month.

4. A method in accordance with claim 3 including the step of adding a seed crystal to said solution.

References Cited UNITED STATES PATENTS 1,137,467 4/1915 Eibr r 23134 1,310,151 7/1919 Bacon 23134 1,896,876 2/1933 Wildman 23134 2,860,952 11/ 1958 Bergeron et al. 23134 3,027,320 3/1962 Buchanan 62-58 3,061,412 10/ 1962 Giordano 23134 3,085,859 4/1963 Scholten et al. 23-134 3,1 15,389 12/1963 Deriaz 23134 3,443,888 5/1969 Calbeck 23--134 OTHER REFERENCES NORMAN YUDKOFF, Primary Examiner -R. T. FOSTER,

Assistant Examiner U.S. Cl. X.R. 

